The binding of oxygen to hemoglobin is thought to be a multistep process with different rate constants for each step. With an all tantalum stopped-flow calorimeter developed earlier, we can easily measure the heat of binding of oxygen to hemoglobin. The all tantalum system will prevent the diffusion of oxygen into the system and will allow us to measure these heats over the entire saturation range, 0-100%; preliminary measurements of the binding heat at 50% saturation are in good agreement with published values.